1. Field of the Invention
Embodiments disclosed herein relate generally to methods for stabilizing shales during drilling. Even more particularly, embodiments disclosed herein relate to methods of using aqueous based wellbore fluids that contain copolymers of cationic comonomers and acrylamide-type comonomers.
2. Background Art
Hydrocarbons are found in subterranean formations. Production of such hydrocarbons is generally accomplished through the use of rotary drilling technology, which requires the drilling, completing and working over of wells penetrating producing formations.
To facilitate the drilling of a well, fluid is circulated through the drill string, out the bit and upward in an annular area between the drill string and the wall of the borehole. Common uses for wellbore fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, fluid used for emplacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
The selection of the type of wellbore fluid to be used in a drilling application involves a careful balance of both the good and bad characteristics of the wellbore fluids in the particular application and the type of well to be drilled. However, historically, aqueous based wellbore fluids have been used to drill a majority of wells. Their lower cost and better environment acceptance as compared to oil based wellbore fluids continue to make them the first option in drilling operations. Frequently, the selection of a fluid may depend on the type of formation through which the well is being drilled.
The types of subterranean formations intersected by a well, include sandstone, limestone, shale, siltstone, etc., many of which may be at least partly composed of clays, including shales, mudstones, siltstones, and claystones. Shale is the most common, and certainly the most troublesome, rock type that must be drilled in order to reach oil and gas deposits. The characteristic that makes shales most troublesome to drillers is its water sensitivity, due in part to its clay content and the ionic composition of the clay.
In penetrating through such formations, many problems may be encountered including bit balling, swelling or sloughing of the wellbore, stuck pipe, and dispersion of drill cuttings. This may be particularly true when drilling with a water-based fluid due to the high reactivity of clay in an aqueous environment. When dry, the clay has too little water to stick together, and it is thus a friable and brittle solid. Conversely, in a wet zone, the material is essentially liquid-like with very little inherent strength and can be washed away. However, intermediate to these zones, the shale is a sticky plastic solid with greatly increased agglomeration properties and inherent strength.
When drilling a subterranean well, as the drill bit teeth penetrate the formation, drill chips are generated by the action of the bit. When these cuttings are exposed to conventional water-based muds, they usually imbibe water and are rapidly dispersed. However recent advances in wellbore fluid technology have developed highly inhibitive muds which appear to reduce the hydration of shale and in doing so produce sticky, plastic shale fragments. These fragments adhere to each other and to the bottomhole assembly and cutting surfaces of the drill bit, gradually forming a large compacted mass of clay on the drilling equipment. This process, or phenomenon, of accumulation and impacting is generally referred to as “balling” or “packing off” of the drilling equipment.
Clay swelling during the drilling of a subterranean well can have a tremendous adverse impact on drilling operations. Bit balling reduces the efficiency of the drilling process because the drillstring eventually becomes locked. This causes the drilling equipment to skid on the bottom of the hole preventing it from penetrating uncut rock, therefore slowing the rate of penetration. Furthermore the overall increase in bulk volume accompanying clay swelling impacts the stability of the borehole, and impedes removal of cuttings from beneath the drill bit, increases friction between the drill bit and the sides of the borehole, and inhibits formation of the thin filter cake that seals formations. Clay swelling can also create other drilling problems such as loss of circulation or stuck pipe and increased viscosity of the wellbore fluid that slow drilling and increase drilling costs. The downtime associated with either soaking the bit or tripping the bit can be very costly and is therefore undesirable.
Thus, given the frequency in which shale is encountered in drilling subterranean wells, there exists a continuing need for methods of drilling using aqueous based wellbore fluids that will reduce potential problems encountered when drilling through shales such as with dispersion of shales, cuttings accretion and agglomeration, cuttings build up, bit balling, and hole cleaning.